1. Field of the Invention
The present invention relates to a heat control method of a thermal head which achieves even printing by controlling the heating state of the thermal head used in printing.
2. Description of the Related Art
A typical thermal head is formed by arranging a plurality of heating elements in one or more rows on a substrate made of, for example, a semiconductor or ceramics, and the heating elements are selectively supplied with electricity to generate heat for printing according to printing information.
According to the construction of such a thermal head, since the temperature of the heating elements and the substrate rises in printing, the current supply time of the heating elements is controlled in consideration of the temperature rise to prevent uneven printing.
In order to control the current supply time, a past data control method and an area control method are conventionally used.
The past data control method controls the current supply time in accordance with the past current supply states of a heating element to be supplied with current and peripheral heating elements.
On the other hand, since the temperature of the substrate gradually rises due to the current supplied to the heating elements, the print density at the beginning of a line becomes different from that at the end of the line. The area control method controls the current supply time so as to eliminate the difference.
However, since the conventional area control method does not appropriately cope with an actual temperature rise of the substrate, it cannot completely prevent print density from being uneven.
More specifically, the conventional area control method finds the total number of heating elements in a line which previously have been supplied with electricity, and adjusts, in accordance with the total number, the degree to which the current supply time for printing of the next line is shortened. Therefore, when the total number is large, the current supply time for the next line is shortened to a great extent.
However, since all the heating elements are uniformly controlled, the same area control is conducted on both heating elements in the upper and lower portions and heating elements in the center in the arrangement direction of the heating elements. As a result, the print density of the upper and lower portions, where heat is apt to escape, is lower than that of other portions.